1. Field of the Invention
This invention relates to a method of converting a construction of a color conversion table for converting colors between different color spaces and a computer-readable recording medium on which a program for converting a construction of a color conversion table is recorded.
2. Description of the Related Art
Generally, a color space represents a coordinate system for representing colors, and various color spaces are available. Such a device as a printer or a scanner has a color space of the device-dependent type which relies upon the device such as a "YMC (Yellow Magenta Cyan)" color space or an "RGB (Red Green Blue)" color space. For example, with a printer which has a minimum value of 0 and a maximum value of 255 as an output signal value when it outputs a color, a color to be outputted is represented (designated) by YMC values within the range from 0 to 255. However, printers of different makers may not necessarily output the same color even if the same YMC values are designated.
Meanwhile, in addition to such color spaces of the device-dependent type as described above, different color spaces "L*a*b*", "XYZ" and so forth of the device-independent type are used widely. Since the color spaces of the device-independent type can provide absolute color representations, they are often used for an intermediate color space when color conversion is performed between color spaces of different devices.
For example, in order to convert an image (data) represented in the "RGB" form read in by a scanner into data represented in the YMC form, the data represented in the "RGB" form are converted into data represented in the L*a*b* form once, and then the data of the L*a*b* form are converted into YMC data.
Here, in order to perform color conversion between different color spaces in this manner, a method which uses a color conversion table which represents corresponding relationships of colors between the color spaces is used widely. Normally, in a conversion method which uses a color conversion table, when a color registered in the color conversion table is to be converted, a corresponding relationship registered in the color conversion table is utilized to effect the conversion, but when a color which is not registered in the color conversion table, interpolation is performed based on data registered in the color conversion table to effect conversion of the color.
For example, when an outputting device such as a printer is handled, a color conversion table constructed from colors of a color space obtained by an ordinary color chart calorimeter can be formed, in a color space of the outputting device, as a grid type conversion table, but in another color space of the device-independent type, it is difficult to form the color conversion table as a grid type conversion table. Therefore, usually a color of an image represented by a color space of the device-independent type is converted. Consequently, conversion for determination of a color in a color space of the device-dependent type from a color in another color space of the device-independent type is important.
However, as well known in the art, when it is tried to determine a color of another color space (refer to an arrow mark B of FIG. 25) by interpolation using a color conversion table constructed from a color space (refer to an arrow mark A of FIG. 25) which has a grid-like distribution, it can be determined at a high speed by simple interpolation calculation, but when it is tried to determine a color of a color space by interpolation using a color conversion table constructed from a color space which does not have a grid-like distribution, high speed interpolation is very difficult.
In order to solve such a subject as just described, such a technique as disclosed in Japanese Patent Laid-Open Application No. Heisei 7-95431 has been proposed. According to the technique, a method called convex interpolation is used so that interpolation may be applied to conversion between data which are not present on grid points in different color spaces, and colors in a space of the device-dependent type corresponding to colors at grid points in another space of the device-independent type are determined in advance by convex interpolation and corresponding relationships between them are registered in a reverse color conversion table (which has a grid-like shape in-the space of the device-independent type) so that, by effecting color conversion processing later using the reverse color conversion table, high speed color conversion may be performed by interpolation of colors of a simple grid-like distribution.
Further, in order to solve the subject described above, another method has been proposed in "Device Independent Color Reproduction and Evaluation of Hardcopy in Scanner-Printer System" in a publication of the Institute of Electronics, Information and Communication Engineers (Technical Report of IEICE, IE92-121). According to the technique, interpolation calculation is performed between colors which are distributed like a grid in a YMC space and those colors which are distributed like a grid in an L*a*b* space are selected from a result of the calculation to produce a grid-like color conversion table in the L*a*b* color space.
Further, for interpolation in a color space which does not have a grid-like distribution as described above, a method wherein interpolation is performed with a weight which increases in reverse proportion to the fourth power of a distance has been proposed already. However, where the method is used, an influence upon interpolation processing called scalloping occurs.
In order to solve the subject just mentioned. such a technique as disclosed in Japanese Patent Laid-Open Application No. Heisei 7-307872 has been proposed. According to the technique, the number of data which construct a color conversion table is increased by linear interpolation and the thus increased data are registered to produce another color conversion table having a larger number of components (having smaller distances between components). In this manner, a color conversion table having a number of data larger than the number of data obtained by actual measurement is produced to reduce the time required for production of a conversion table and allow color conversion with a high degree of accuracy.
However, the techniques described above individually have the following subjects to be solved.
First, the technique disclosed in Japanese Patent Laid-Open Application No. Heisei 7-95431 has a subject in that, since colors in a color space of the color-dependent type corresponding to colors at grid points in another color space of the device-independent type are determined by convex interpolation to produce a reverse color conversion table, a large amount of calculation is required to produce the reverse color conversion table and remarkably much time is required.
It is examined here, for example, to convert a color conversion table which has a grid-like distribution in a YMC space of the outputting device-dependent type and does not have a grid-like distribution in an L*a*b* space of the device-independent type into another color conversion table which has a grid-like distribution in the L*a*b* space. However, since normally the number of data of a color conversion table is approximately several hundreds, it is assumed that the first-mentioned color conversion table has 729 colors in the YMC color space which has 9 gradations for each of Y, M and C and also the color conversion table to be produced newly which has a grid-like distribution in the L*a*b* space has 729 colors.
In this instance, with the technique described above, in order to derive a plurality of (at least four) points which are at shortest distances to each point to be obtained by interpolation, for each of colors (729 colors) at L*a*b* grid points to be produced by interpolation, a plurality of points must be selected from within different colors (729 colors) which are present originally and have a non-grid-like distribution in the L*a*b* space. Accordingly, with the present technique, calculation of a distance must be performed by 729.times.729.times.4 times, and also complicated discrimination of convex interpolation must be performed by 729 times.
Meanwhile, with the technique disclosed in the publication of the Institute of Electronics, Information and Communication Engineers (Technical Report of IEICE, IE92-121) mentioned hereinabove, since the CBS (Cubic Binary Search) method which calculates a point nearest to a grid point at a high speed cannot be applied because color data (samples) are distributed at random on coordinate (grid) systems on both of the input and output sides of a conversion system of an inputting device (imaging system), there is a subject that much time is required for processing for color conversion.
Further, the technique has another subject that, also in a system other than such an imaging system as mentioned above, since the number of colors to be produced by interpolation calculation is determined without taking a manner of looking of a color (a difference in color in an L*a*b* space or the like) from a human being into consideration, a large number of colors very close to one another are sometimes produced and a sufficiently high processing speed cannot be achieved.
On the other hand, with the technique disclosed in Japanese Patent Laid-Open Application No. Heisei 7-307872, while it is possible to reduce the influence of scalloping to cause color data to be distributed regularly (to produce a regular interval dispersion of colorimetry signals) in a space of the device-dependent type by increasing the number of color data which construct a color conversion table, it is very difficult to cause color data to be distributed regularly in a similar manner in a space of the device-independent type. Consequently, the technique has a subject in that an easy interpolation method such as three line interpolation (eight point cube interpolation) which can be applied in regard to a grid-like color conversion table cannot be used in a space of the device-independent type.